There are many medical diagnostic tests that require a fluid, for example, blood (sometimes referred to as whole blood), serum, plasma, cerebrospinal fluid, synovial fluid, lymphatic fluid, calibration fluid, and urine. With respect to blood, a blood sample is typically withdrawn in either an evacuated tube containing a rubber septum, or a syringe, and sent to a central laboratory for testing. The eventual transfer of blood from the collection site to the testing site results in inevitable delays. Moreover, the red blood cells are alive and continue to consume oxygen during any delay in testing, which in turn changes the chemical composition of the blood sample, from the time the blood sample is collected to the time the blood sample is analyzed, measured or tested.
One example of a blood analysis technique that is affected by delay in testing and transfer of blood from the blood collection device to the analyzer, is CO-oximetry. CO-oximetry is a spectroscopic technique that is used to measure the different Hemoglobin (Hb) species present in a blood sample, for example, Oxy-Hb, Deoxy-Hb, Met-Hb, Carboxy-Hb and Total-Hb. Some Co-oximeters can also measure Sulf-Hb and Fetal-Hb. The results of CO-oximetry is used to provide Hb Oxygen Saturation (sO2) measurements in two ways: 1) Functional sO2 is defined as the ratio of Oxy-Hb to the sum of Oxy-Hb and Deoxy-Hb; and 2) Fractional sO2 is defined as the ratio of Oxy-Hb to the Total-Hb.
If the blood sample is exposed to air, the sO2 measurements may become falsely elevated, as oxygen from the air is absorbed into the blood sample. CO-oximetry usually requires hemolyzing the red blood cells (hemolysis) using a sound generator, in order to make the blood sample more transparent for spectroscopic measurement; blood with intact red cells scatter significantly more electromagnetic radiation (EMR) than hemolyzed blood. Hemolysis can also be accomplished by mixing a chemical for example a detergent, with the blood. Parameters that can be measured in blood by spectroscopic techniques (or spectroscopy, sometimes referred to as spectrometry) are limited by the amount of EMR absorbed by the analytes measured. In contrast, for example without limitation, hydrogen ions (which determine pH) and electrolytes (e.g. sodium, potassium, and chloride) do not absorb EMR in the approximate wavelength range of about 300 nm to 2500 nm. Therefore, if this wavelength range is used to conduct spectroscopic measurements of Hb species, then these important parameters, i.e., hydrogen ions and electrolytes, must be measured by another means.
Another example of a blood analysis technique that is affected by the aforementioned sources of error is blood gases. Traditionally, blood gas measurement includes the partial pressure of oxygen (pO2), the partial pressure of carbon dioxide (PCO2), and pH. From these measurements, other parameters can be calculated, for example, sO2, bicarbonate, base excess and base deficit. Blood gas and electrolyte measurements usually employ biosensors, also referred to as electrochemical sensors or electrochemical detectors. Bench-top analyzers are available, which perform the following: (1) measurement of blood gases, (2) CO-oximetry, or (3) measurement of blood gases and CO-oximetry. Some combinations of diagnostic measurement instruments also include electrolytes, and other measurements for example lactate and creatinine. Because these instruments are large and expensive, they are usually located in central laboratories. Biosensor technology is also limited by the blood parameters biosensors can measure. To the inventor's knowledge, biosensors are not currently available for performing CO-oximeters. U.S. Pat. Nos. 5,096,669 and 7,094,330 to Lauks et al, as examples, describe in details cartridges that employ biosensor technology for POCT. In particular, they teach about pH measurement (a potentiometric measurement), blood gas measurement (a potentiometric and an amperometric measurement for pCO2 and pO2 respectively), and hematocrit measurement (a conductivity measurement). U.S. Pat. No. 7,740,804 to Samsoondar (the present inventor) teaches disposable cartridges for spectroscopic measurement (e.g. CO-oximetry) for POCT using unaltered blood. U.S. Pat. Nos. 5,430,542 and 6,262,798 to Shepherd describes a method for making disposable cuvettes having a pathlength in the range of 80 to 130 micrometers for performing CO-oximetry measurement on unaltered blood.
Blood tests for assessing a patient's oxygenation and acid-base status may include pH, sO2, CO2, and Total Hb. The leading POCT analyzers used to assess a patients acid-base status estimate sO2 from a measured partial pO2, and estimate Total Hb from a measured hematocrit. Both hematocrit and pO2 are measured using biosensors.
sO2 calculated from pO2 is criticized in the literature because: 1) pO2 measures the O2 dissolved in the blood plasma, which accounts for only about 1% of the total oxygen in blood—the remaining 99% of blood oxygen is bound to Hb; 2) it is assumed that the patient's red blood cells (RBC) contain normal levels of 2,3-diphosphoglycerate; and 3) the patient has normal levels of dyshemoglobins e.g., Carboxy-Hb and Met-Hb. Dyshemoglobins are onn-functional Hbs. Temperature and pH which are also sources of error are usually corrected for.
Total Hb estimated from hematocrit measurement by conductivity is criticized in the literature because: 1) a certain RBC Hb concentration is assumed for all patients; 2) alteration in plasma protein, electrolytes, white cells, and lipids are sources of errors in hematocrit measurement. These assumptions can lead to significant errors in managing seriously ill patients. Moreover, Hb measurement is preferred over hematocrit measurement for evaluating chronic anemia and blood loss. Unnecessary blood transfusion due to underestimation of Hb from hematocrit is a major concern.
In choosing a POCT analyzer, a user must understand clearly the parameters that are actually measured and the parameters that are calculated from measured parameters. Measurement of Total Hb and sO2 performed by spectroscopy provide the best measurement of a patient's oxygenation status, because they are more accurate than results calculated from hematocrit and pO2 respectively. Lab analyzers can easily combine biosensor and spectroscopic technologies because analyzer size is not a limitation. Currently, no small POCT analyzer is available that provides blood gases (includes pH) and CO-oximetry. Some POCT vendors provide a solution in the form of a separate POCT analyzer just for performing CO-oximetry, which complements their blood gas POCT analyzer.
Since CO-oximetry measures functional Hb species, and non-functional Hb species like Carboxy-Hb and Met-Hb, a physician can continue to confidently monitor a patient's oxygenation status non-invasively using a Pulse Oximeter. According to best practice, pulse oximetry should only be used after verifying that the patient's blood does not contain significant amount of non-functional Hb. The presence of elevated non-functional hemoglobin is a source of error in pulse oximetry. The present invention can use capillary blood as well as arterial blood, which provides a major advantage for babies. Obtaining arterial blood is painful, must be performed by a qualified person like a physician, and the resulting blood loss in babies is clinically significant. This cartridge of the present invention will also facilitate monitoring Met-Hb in neonates during treatment with nitric oxide for respiratory distress, and facilitate measuring bilirubin for assessing neonatal jaundice. The use of capillary blood also makes the present invention an attractive tool for monitoring sO2, Carboxy-Hb (increased due to carbon monoxide poisoning resulting from smoke inhalation) and pH in firefighters and other victims of smoke inhalation. Most of these victims will be treated with oxygen, which elevates the pO2, therefore pO2 cannot be used to assess the blood oxygen content. CO-oximetry is therefore essential to victims of smoke inhalation. Capillary blood is usually obtained from a finger, heel or ear lobe prick. The capillary blood can be altered to more closely resemble arterial blood by applying a heating pad to the site that will be pricked.
U.S. Pat. No. 8,206,650 to Samsoondar (the present inventor) teaches the combination of spectroscopy and biosensor technologies in one disposable cartridge, and can therefore provide pH, blood gases and CO-oximetry on a small POCT analyzer. The users are provided with the convenience of applying the sample once, as opposed to using a first analyzer that employs biosensor technology alone, and a second analyzer that employs spectroscopy alone. However, U.S. Pat. No. 8,206,650 does not provide details required by a person with ordinary skill in the art, for making a functional cartridge, and further does not provide details that can be applied to a cartridge manufacturing process.
U.S. Pat. No. 8,206,650 provides a single cartridge option that can be used to test blood from a syringe like arterial blood, and capillary blood at the surface of a body part, which is a very important consideration when the patient is a neonate. However, the option for obtaining capillary blood is limited. A person of ordinary skill in the art of blood gases will appreciate that the pO2 will be overestimated significantly due to atmospheric contamination; current practice includes inserting the open of a capillary tube inside the drop of blood, quickly sealing the ends of the capillary tube, and taking the sample to an analyzer.
U.S. Pat. No. 8,206,650 teaches the use of an air chamber/bladder to force blood from an optical chamber into a biosensor conduit, but it does not teach any means for mitigating blood flow into the air bladder when the optical chamber receives the blood from the cartridge inlet. Since blood is very precious, especially from a baby, it is not desirable that any of the blood should be wasted. It is possible that when blood is drawn into the cartridge taught in U.S. Pat. No. 8,206,650, blood could at least enter the conduit connecting the air chamber with the inlet chamber. This blood will not contribute to filling the biosensor conduit for biosensor measurements. Other limitations of the cartridge described in U.S. Pat. No. 8,206,650 will become apparent as the various embodiments of the present invention are described.